To obtain further information on the role of hepatic HMG CoA reductase in the synthesis of the increased amount of cholesterol in genetically obese rats compared to their non-obese siblings, it is proposed that additional assays of the enzyme be made at intervals during the first 6 hours of feeding. Such a comparison of the diurnal cycle of this enzyme would provide a more complete picture than the single point comparison already obtained. Measurement of carcass cholesterol as well as blood and liver cholesterol concentrations would provide information on the pool size of cholesterol.